Poorly looking plum tree

I have a 3 year old victoria plum tree and it's looking very poorly at the moment.  I noticed that the leafs were covered in whitefly/greenfly after all the rain we had and sprayed it with a soapy mixture.  Now the leafs are curling up and looking dead and the whole tree seems to be invaded by fly's as well as whitefly.  Can anyone suggest what to do, pruning and spraying wise so I can prevent this happening next year.

Soapy spray is fine for a small infestation-but I feel you are going to have to call on the big boys-I would invest in a proprietary spray suitable for edible crops and keep repeating- white-fly and all aphids breed at an alarming rate

Geoff is right - get yourself a proprietary spray and use according to the instructions. Pruning will have absolutely no effect at deterring insects.